The present invention relates to apparatus and method of aligning the electron gun system of a multi-cathode-ray tube, in particular of a color television picture tube, with respect to the phosphor dots on the screen-plate thereof, in which the electron gun system as compsed of several guns, as well as the picture tube, prior to the assembly, are separated from one another and positioned in a sealing device, and in which the electron gun system is supported in this sealing device capable of being turned about its longitudinal centerline axis.
Present color tv picture tubes have a display screen with phosphor dots emitting light in the three primary colors, red, green and blue, and, in the proximity thereof, a color selecting electrode extending substantially parallel in relation thereto. The phosphor dots are deposited, for example, in the course of a photochemical process, and their positions on the screen-plate are exactly defined with respect to the axes of the individual systems or electron gun associated therewith. The landing points of the electron beams on the screen-plate must exactly coincide with the phosphor dots associated therewith.
In the case of color tv picture tubes in which the electron guns are arranged, e.g., in a triangle (so-called delta tubes) the electron gun system must be fixed in such a way inside the neck portion of the picture tube that it is turned by less than 3.degree. with respect to the position of the phosphor dots on the screen-plate.
In the case of color television tubes in which the electron guns are arranged in one line next to each other (in-line arrangement), and in which there are not used, e.g., any special dynamic convergence means, it is necessary to provide for a more exact position definement.
In one conventional method used in connection with color tv picture tubes employing a delta arrangement of the electron guns, the contact pins in the pressed glass base are used as marks for aligning the electron gun system. These contact pins are connected by metal wires or metal tapes to the electron guns, so that the electron gun system forms one structural unit together with the pressed glass base. It can easily happen, however, that the thin metal wires or metal tapes are bent, so that there is caused a misalignment of the system with respect to the pressed glass base and, consequently, also with respect to the phosphor dots on the screen-plate, with this misalignment being greater than admissible.
The German Offenlegungsschrift (DT-OS) No. 23 54 338 (U.S. Pat. application Ser. No. 302,587, filed 10/31/72 of J. F. Segro and G. L. Fassett, RCA); discloses a method according to which the electron gun system is aligned with the aid of a reference plane extending through the center points of at least two passage holes for the electron beams in an accelerating and focusing grid. This grid is composed of two cap-shaped parts meeting against each other with their bottom parts. In these bottom parts there are provided the passage holes for the electron beams. The cap-shaped parts of the grid for use with in-line picture tubes have, e.g., flattened side portions extending in parallel with the plane extending through the center axes of the electron guns. Two mechanical gauges in the form of displaceable rods are brought into contact with one of these side parts when both the electron gun system and the picture tube are positioned separately from one another in the sealing device. These gauges serve to measure the distance of the contacting points at the side part from a reference plane representing the ideal position of the electron gun system with respect to the screen-plate. By a comparator it is made possible to make the two measured distances alike by turning the electron gun system about its longitudinal centerline axis. The viewing plane as fixed by the two points of contact, will then be in parallel with the reference plane. The tolerance of turning the electron gun system is at about 0.5.degree. or below.
The disadvantage of such a mechanical alignment is seen in that by the application of the mechanical gauges to the electron gun system, the latter can be easily twisted (turned) or bent and damaged. In order to be able to perform an exact measurement, the two gauges must be at a large as possible lateral spacing from one another. Owing to a possible parallel offset of the pressed glass base and the electron gun system with respect to one another, when assembled, it may easily happen that one of the gauges will come in touch with the rim portion on the flat side portion of the grid, or even beyond the latter, which may falsify the measurement and, under certain circumstances, may even cause the electron gun system to be damaged.